A liquid crystal panel driver LSI and a CCD driver LSI, for example, are operated at a power supply voltage of 10V or higher, and therefore high breakdown voltage transistors having a breakdown voltage of 20V or higher are normally required. On the other hand, low breakdown voltage transistors are used in internal control logic sections that need to be small in size and operated at high speeds. Wells where high breakdown voltage transistors are formed tend to be made deeper in order to secure the well breakdown voltage. In contrast, wells where low breakdown voltage transistors are formed tend to be made shallower in order to reduce the element size and to achieve higher speeds. For this reason, high breakdown voltage transistors are formed in a chip that is different from a chip for low breakdown voltage transistors, and are conventionally formed as an externally mounted circuit.
In view of the foregoing, one object of the present invention is to provide a method for manufacturing a semiconductor device having a high breakdown voltage transistor and a low breakdown voltage transistor with different driving voltages in a common substrate.